As described in the Japanese Patent No. 2576301, an image forming apparatus may change its image forming speed without changing the rotational speed of a rotatable polyhedron deflector such as a polygon mirror. For example, thinning processing may be applied to reduce the image forming speed.
As illustrated in FIG. 1, thinning processing may be performed by thinning a synchronous signal such that one surface out of a predetermined number of surfaces of the polygon mirror is used for image forming. FIG. 1(a) illustrates a timing chart for explaining the time for outputting a plurality of synchronous signals for a plurality of image forming stations 1ST, 2ST, 3ST, and 4ST when the image forming apparatus performs image formation at a default speed. FIG. 1(b) illustrates a timing chart for explaining the time for outputting a plurality of synchronous signals when the image forming speed is reduced by half through the thinning processing. While the thinning processing allows the image forming speed to decrease without changing the rotational speed of the polygon mirror, it may cause the positional shift in color images respectively generated at the image forming stations 1ST, 2ST, 3ST, and 4ST such that the resultant full color image may have lower image quality. For example, as indicated by “A” in FIG. 1(b), when the synchronous signal for one image forming station 2ST is thinned out without considering synchronization with respect to the synchronous signals output for other image forming stations 1ST, 3ST, and 4ST, the synchronous signal that has been thinned out is not in synchronization with the other synchronous signals.
FIG. 1(c) is a timing chart for explaining the time at which image data is output. In addition to the synchronous signals 1ST to 4ST illustrated in FIG. 1(b), a plurality of gate signals 1ST GATE, 2ST GATE, 3ST GATE, and 4ST GATE is output. Referring to FIG. 1(c), the image data for a specific color is output when a predetermined time period passes after the synchronous signal for the corresponding one of the image forming stations 1ST, 2ST, 3ST, and 4ST is output. When the synchronous signal 2ST for the image forming station 2ST is not in synchronization with the synchronous signals for the other image forming stations 1ST, 3ST, and 4ST, the time for outputting the image data for the image forming station 2ST will be out of phase with respect to the time for outputting the image data for the other image forming stations 1ST, 3ST, and 4ST as indicated by “B” in FIG. 1. This causes the image data formed by the image forming station 2ST to be shifted by one line in the sub-scanning direction with respect to the image data formed by the other image forming stations 1ST, 3ST, and 4ST, thus causing the resultant full color image to have lower image quality.